Optical fiber and method of manufacturing optical fiber

ABSTRACT

An optical fiber having excellent strength that can be manufactured at low cost, as well as a method for making such optical fiber, is provided. An optical fiber  1  is a silica-based optical fiber comprising a core  11 , an optical cladding  12  surrounding the core  11 , and a jacketing region  13  surrounding the optical cladding  12  and having a uniform composition throughout from the internal circumference to the outer circumference. A compressive strained layer having a residual compressive stress is provided at the outermost circumference of the jacketing region  13.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber and a method ofmanufacturing the optical fiber.

2. Description of the Background Art

In accordance with the development of FTTH (Fiber To The Home),improvement in optical fiber installation efficiency has been inprogress. In the installation of a FTTH system, it is often necessary towire an optical fiber in a narrow duct or at a place available only fora small radius of curvature. Thus, optical fibers which do not causelight leakage even if they are bent have been developed, so that theycan play a role of helping to enhance the efficiency of suchinstallation. For example, there is a case in which an optical fiber issupposed to be bent at a radius of curvature of 5 mm for installation ofa FTTH system, and accordingly an optical fiber which can allow suchbending radius of curvature has been sought.

On the other hand, it is known that the long-term reliability of anoptical fiber decreases when the optical fiber is subjected to bendingat a small radius of curvature. Therefore, some techniques have beenattempted such that the rigidity of a cable is strengthened to preventoptical fibers from bending substantially at a small radius of curvatureor such that the elongation percentage of an optical fiber during ascreening is made larger to enhance long-term reliability. However, bothof such techniques not only result in increase of cost, but also becomeeither a cause for decrease in the ease of handling optical fibers as aresult of enhancing the rigidity of the cable or a cause for decrease inthe strength of glass constituting the optical fiber as a result ofincreasing the tensile stress of such screening.

A method for enhancing the strength of an optical fiber itself isdisclosed in Japanese Patent Application Publication No. H 2-27308.According to the method, the outer surface of the optical fiber iscoated with carbon. Since such carbon coating is done using ahydrocarbon gas, additional facilities for exhausting the gas will beneeded to manufacture such optical fibers. Also, it is difficult toachieve adherence between a carbon layer and a resin layer coated overthe carbon layer. If a colored layer provided for identification has alight color, the colored layer will bear a darkish color because thecarbon layer is black, and consequently the identification function ofthe colored layer will be degraded. Moreover, in order to ensureuniformity in the carbon coating over the entire length, it is necessaryto monitor the condition of the coating layer by performing anadditional inspection, for example such measurement of electricparameters using the conductivity of carbon layer as mentioned in thespecification of U.S. Pat. No. 5,057,781.

Also, Japanese Patent Application Publication No. H4-65327 and JapanesePatent Application Publication No. H5-124831 disclose a method forenforcing glass by providing a glass layer, such as TiO₂-doped SiO₂glass or F-doped SiO₂ glass whose viscosity is lower than that of SiO₂,so that the glass surface of an optical fiber is transformed into acompressive stress layer. However, with such a glass whose compositionis significantly different from SiO₂, there is a case in which thediffusion of hydrogen easily tends to progress such that the networkstructure of glass is cut, resulting in degradation of breaking strengthof glass in the long run because of decrease in the strength of thenetwork structure of glass. Also, it is difficult to obtain desiredoptical properties since the refractive index differs from the value ofSiO₂ because the composition is different. Besides, it will cause a costincrease because of an additional process needed for providing a layerhaving another different composition over the outermost layer.

The above-mentioned methods for enhancing the strength of an opticalfiber itself, either the method of applying a carbon coating around theoutside surface of glass or the method of making the outside surface ofglass to be a glass having a viscosity lower than SiO₂, were bothdisadvantageous in terms of quality and manufacturing cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical fiber havingexcellent strength that can be manufactured at low cost. Another objectof the invention is to provide a method for making such optical fiber.

An embodiment provided to achieve such object is a silica-based opticalfiber which includes a core, an optical cladding surrounding the core,and a jacketing region that surrounds the optical cladding and that hasa uniform composition throughout from the internal circumference to theouter circumference, wherein a compressive strained layer having aresidual compressive stress is provided at the outermost circumference.

The stress of the compressive strained layer in the optical fiber of thepresent invention is preferably a compressive stress of 10 MPa or more.Preferably, the compressive strained layer has a thickness that isequivalent to 30% or less of the outer diameter of the jacketing region.The static fatigue coefficient is preferably 20 or more. It ispreferable that the breaking strength be 400 kgf/mm² or more when atensile rate of 5 mm/min is applied. It is also preferable that theratio (A490/A800) of 490 cm⁻¹ peak area A490 to 800 cm⁻¹ peak area A800of Raman scattering spectrum at the outermost circumferential portion ofthe jacketing region be smaller than the ratio (A490/A800) at the core.The circumferential deviation of the compressive stress is preferablyequal to or less than 10 MPa.

Another embodiment of the present invention is a method of manufacturingan optical fiber, comprising: (1) a fiber drawing step in which a glassfilament having a desired outer diameter is formed by heating and fusingone end of a silica-based optical fiber preform that includes a core, anoptical cladding surrounding the core, and a jacketing regionsurrounding the optical cladding; and (2) a stress imparting step inwhich the outer circumferential portion of the glass filament isre-heated to a temperature higher than a glass transition point afterthe temperature of the whole glass filament formed in the fiber drawingstep has become lower than the glass transition point, and thereby acompressive strained layer is formed at the outermost circumferentialportion of the jacketing region.

At the stress imparting step in the optical fiber manufacturing methodaccording to the present invention, a glass filament may be heated bymeans of irradiation of laser beams output from one or more lasers, orthe glass filament may be heated by an annular heating furnacesurrounding the glass filament as its central axis, or the glassfilament may be heated using a burner. Also, it is preferable to twist aglass filament in alternate directions around its central axis at thefiber drawing step. In addition, the deviation of temperature in theouter circumference of the glass filament during heating at the stressimparting step is preferably less than 50° C. Also, L1/V is preferably0.003 second or more, and L2/V is preferably 1 second or less, where Vis a line speed of the optical fiber, L1 is a length extending to thestress imparting unit from the position at which the outer diameter ofthe glass filament is decreased to 105% or less of the desired outerdiameter, and L2 is the length of the stress imparting unit. The tensionapplied to the optical fiber glass at the stress imparting step ispreferably 25 gms or more.

The optical fiber according to the present invention has excellentstrength and can be manufactured at low cost. Also, the manufacturingmethod of the present invention enables manufacturing high-strengthoptical fibers at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical fiber relating to Embodiment 1 of the presentinvention: region (a) is a sectional view thereof; and region (b) is arefractive index profile thereof.

FIG. 2 shows an optical fiber relating to Embodiment 2 of the presentinvention: region (a) is a sectional view thereof; and region (b) is arefractive index profile thereof.

FIG. 3 shows an optical fiber relating to Embodiment 3 of the presentinvention: region (a) is a sectional view thereof; and region (b) is arefractive index profile thereof.

FIG. 4 is a sectional view illustrating an optical fiber relating toEmbodiment 4 of the present invention.

FIG. 5 is a graph showing examples of stress profiles of optical fibersrelating to Embodiment 2, as well as a stress profile of Comparativeexample.

FIG. 6 is a conceptional schematic diagram illustrating an embodiment ofoptical fiber manufacturing method of the present invention.

FIG. 7 shows a first example of stress imparting unit used at the stressimparting step in the optical fiber manufacturing method relating to thepresent invention: region (a) is a top view thereof; and region (b) is aside view thereof;

FIG. 8 shows a second example of stress imparting unit used at thestress imparting step in the optical fiber manufacturing method relatingto the present invention: region (a) is a top view thereof; and region(b) is a side view thereof.

FIG. 9 shows a top view of a third example of stress imparting unit usedat the stress imparting step in the optical fiber manufacturing methodrelating to the present invention.

FIG. 10 shows a fourth example of stress imparting unit used at thestress imparting step in the optical fiber manufacturing method relatingto the present invention: region (a) is a top view thereof; and region(b) is a side view thereof.

FIG. 11 shows a top view of a fifth example of stress imparting unitused at the stress imparting step in the optical fiber manufacturingmethod relating to the present invention.

FIG. 12 is a graph illustrating Raman scattering spectrum of silicaglass and 800 cm⁻¹ peak area A800 and 490 cm⁻¹ peak area A490 of thespectrum.

FIG. 13 is a graph showing the ratio (A490/A800) at each position in asection of an glass fiber, in the case where the heat treatment of aglass filament is performed and in the case where the heat treatment ofa glass filament is not performed, respectively.

FIG. 14 is a graph showing the relationship between a residual stressand a breaking strength in an optical fiber.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed in reference to the accompanying drawings. The drawings areprovided for the purpose of explaining the embodiments and are notintended to limit the scope of the invention. In the drawings, anidentical mark represents the same element so that the repetition ofexplanation may be omitted. The dimensional ratios in the drawings arenot always exact.

FIG. 1 shows an optical fiber 1 relating to Embodiment 1 of the presentinvention: region (a) is a sectional view thereof; and region (b) is arefractive index profile thereof. The optical fiber 1 is a silica-basedoptical fiber and includes a core 11, an optical cladding 12 thatsurrounds the core 11, a jacketing region 13 which surrounds opticalcladding 12, and a protective coating layer which surrounds thejacketing region 13 (not shown in FIG. 1). The refractive index of thecore 11 is higher than the refractive index of the optical cladding 12.The refractive index of jacketing region 13 may be either the same as ordifferent from that of the optical cladding 12.

The refractive index may be uniform or varied in a radial direction inthe core 11 and the optical cladding 12, respectively. There areoccasionally cases where appropriate additives for increasing ordecreasing the refractive index are added to the core 11 and the opticalcladding 12, respectively. The respective concentration of the additivesin the core 11 and the optical cladding 12 may be made non-uniformintentionally or by an unintentional manufacturing variation.

The jacketing region 13 is a region to which the electric field ofcore-guided mode light hardly reaches and which does not have anyinfluence on the optical properties of the optical fiber 1. For example,the inside diameter of the jacketing region 13 (the outside diameter ofthe optical cladding 12) is equal to or more than three times the modefield diameter of the core-guided mode light. The jacketing region 13 issubstantially uniform in composition from the internal circumferentialpart to the outer circumferential part. A compressive strained layer inwhich the compressive stress remains is formed at the outermostcircumferential portion of the jacketing region 13.

In the optical fiber 1, the outermost circumferential portion of thejacketing region 13 having a substantially uniform composition is formedas a compressive strained layer, so that the optical fiber 1 hasexcellent strength, without causing such problems as caused by providinga conventional carbon coating or glass having different viscosity aroundthe outer circumference thereof. Also, the optical fiber 1 according tothe present invention can be manufactured at a low cost as in the caseof a conventional optical fiber.

FIG. 2 shows an optical fiber 2 relating to Embodiment 2 of the presentinvention: region (a) is a sectional view thereof; and region (b) is arefractive index profile thereof. The optical fiber 2 is a silica-basedoptical fiber and includes a core 21, a depressed region 22 surroundingthe core 21, a cladding 23 surrounding the depressed region 22, ajacketing region 24 surrounding the cladding 23, and a protectivecoating layer which surrounds the jacketing region 24 and is not shownin FIG. 2.

The optical cladding consists of the depressed region 22 and thecladding 23. When the refractive index of the core 21 is n21, and therefractive index of the depressed region 22 is n22, and the refractiveindex of the cladding 23 is n23, there is a relationship of n21>n23>n22with respect to these refractive indexes. The refractive index of thejacketing region 24 may be the same as or different from the refractiveindex of the cladding 23.

The core 21, the depressed region 22, and the cladding 23 may have auniform refractive index in a radial direction, respectively, or theirrespective refractive index may vary in a radial direction. There arecases in which appropriate additives for increasing or decreasing therefractive index are added to the core 21, the depressed region 22, andthe cladding 23, respectively. In such cases, the concentration of theadditives of the core 21, the depressed region 22, and the cladding 23may be made non-uniform intentionally or by an unintentional variationof manufacture.

The jacketing region 24 is a region to which the electric field ofcore-guided mode light hardly reaches and which does not have aninfluence on the optical properties of the optical fiber 2. For example,the inside diameter of the jacketing region 24 (the outside diameter ofthe cladding 23) is equal to or more than three times the mode fielddiameter of core-guided mode light. The jacketing region 24 has asubstantially uniform composition from the internal circumferential partto the outer circumferential part. A compressive strained layer in whicha compressive stress remains is formed at the outermost circumferentialportion of the jacketing region 24.

In the optical fiber 2, the outermost circumferential portion of thejacketing region 13 having a substantially uniform composition is formedas a compressive strained layer, so that the optical fiber 2 also hasexcellent strength, without causing such problems as caused by providinga conventional carbon coating or glass having different viscosity aroundthe outer circumference thereof. Also, the optical fiber 2 according tothe present invention can be manufactured at a low cost in substantiallythe same method as a method of manufacturing an ordinary optical fiber.

FIG. 3 shows an optical fiber 3 relating to Embodiment 3 of the presentinvention: region (a) is a sectional view thereof; and region (b) is arefractive index profile thereof. The optical fiber 3 is a silica-basedoptical fiber and includes a first core 31, a second core 32 surroundingthe first core 31, a depressed region 33 surrounding the second core 32,a cladding 34 surrounding the depressed region 33, a jacketing region 35surrounding the cladding 34, and a protective coating layer whichsurrounds the jacketing layer 35 and is not shown in FIG. 2.

The optical cladding consists of the depressed region 33 and thecladding 34. When the refractive index of the first core 31 is n31, andthe refractive index of the second core 32 is n32, and the refractiveindex of the depressed region 33 is n33, and the refractive index of thecladding 34 is n34, there is a relationship of n31>n32>n34>n33 withrespect to these refractive indexes. The refractive index of thejacketing region 35 may be the same as or different from the refractiveindex of the cladding 34.

The first core 31, the second core 32, the depressed region 33, and thecladding 34 may have a uniform refractive index or different refractiveindexes in a radial direction, respectively. There are cases in whichappropriate additives for increasing or decreasing the refractive indexare added to the first core 31, the second core 32, the depressed region33, and the cladding 34, respectively. In such cases, the concentrationof the additives of the first core 31, the second core 32, the depressedregion 33, and the cladding 34 may be made non-uniform intentionally orby an unintentional variation of manufacture.

The jacketing region 35 is a region to which the electric field ofcore-guided mode light hardly reaches and which does not have aninfluence on the optical properties of the optical fiber 3. For example,the inside diameter of the jacketing region 35 (the outside diameter ofthe cladding 34) is equal to or more than three times the mode fielddiameter of core-guided mode light. The jacketing region 35 has asubstantially uniform composition from the internal circumferential partto the outer circumferential part. A compressive strained layer in whicha compressive stress remains is formed at the outermost circumferentialportion of the jacketing region 35.

Also in the optical fiber 3, the outermost circumferential portion ofthe jacketing region 35 having a substantially uniform composition isformed as a compressive strained layer, so that the optical fiber 3 hasexcellent strength, without causing such problems as caused by providinga conventional carbon coating or glass having different viscosity aroundthe outer circumference thereof. Also, the optical fiber 3 according tothe present invention can be manufactured at a low cost in substantiallythe same method as a method of manufacturing an ordinary optical fiber.

FIG. 4 is a sectional view illustrating an optical fiber 4 relating toEmbodiment 4 of the present invention. The optical fiber 4 is asilica-based holey fiber and includes a core 41, an optical cladding 42surrounding the core 41, a jacketing region 43 surrounding the opticalcladding 42, and a protective coating layer which surrounds thejacketing layer 43 and is not shown in FIG. 3. In the optical cladding42, holes 44 are formed at vertexes of a regular hexagonal shape at aposition in the main medium having the same composition as the core 41such that the core 41 lies at the center of the hexagonal shape, wherebythe effective refractive index of the optical cladding 42 is smallerthan the refractive index of the core 41. The refractive index of thejacketing region 43 may be either the same as or different from therefractive index of the main medium of the optical cladding 42.

The jacketing region 43 is a region to which the electric field ofcore-guided mode light hardly reaches and which does not have aninfluence on the optical properties of the optical fiber 4. For example,the inside diameter of the jacketing region 43 (the outside diameter ofthe cladding 42) is equal to or more than three times the mode fielddiameter of core-guided mode light. The composition of the jacketingregion 43 is substantially uniform from the internal circumferentialpart to the outer circumferential part. A compressive strained layer inwhich a compressive stress remains is formed at the outermostcircumferential portion of the jacketing region 43.

In the optical fiber 4, the outermost circumferential portion of thejacketing region 43 having a substantially uniform composition is formedas a compressive strained layer, so that the optical fiber 4 hasexcellent strength, without causing such problems as caused by providinga conventional carbon coating or glass having different viscosity aroundthe outer circumference thereof. Also, the optical fiber 4 according tothe present invention can be manufactured at a low cost substantially inthe same method as a method of manufacturing a usual holey opticalfiber.

Optical fibers according to the present invention are not limited tothose of Embodiments 1 to four. The optical fibers according to theinvention may have any arbitrary refractive index profile at otherregions, provided that the composition of their jacketing region issubstantially uniform from the internal circumferential part to theouter circumferential part and that a compressive strained layer inwhich a compressive stress remains is formed at the outermostcircumferential portion of the jacketing region.

FIG. 5 is a graph showing three examples of stress profiles of anoptical fiber 2, together with a stress profile of Comparative examplewhich differs from the optical fiber 2 only in that no compressivestrained layer is formed at the outermost circumferential portion of thejacketing region 13. In the optical fiber of Comparative example, atensile stress remains at the vicinity of the outermost circumferentialportion of the jacketing region. On the other hand, in the optical fiber2, a compressive strained layer in which the compressive stress remainsis formed at the outermost circumferential portion of the jacketingregion. The optical fiber 2 has excellent strength because of suchcompressive strained layer.

The residual compressive stress in the compressive strained layer of thejacketing region is preferably 10 MPa or more, and thereby the glassstrength can advantageously be enhanced. Preferably, the residualcompressive stress in the compressive strained layer of the jacketingregion is 30 MPa or more, and more preferably 100 MPa or more.

The thickness of the compressive strained layer of the jacketing regionis preferably 30% or less relative to the outside diameter of thejacketing region, whereby the tensile strain afforded inside can be keptsmall, enhancing the glass strength. Also, it becomes easy to afford acompressive stress of 10 MPa or more to the outermost circumferentialportion of the jacketing region.

Even in the case where a compressive strained layer in which acompressive stress remains is formed at the outermost circumferentialportion of the jacketing region, it would be difficult to enhance thelong-term reliability of the optical fiber if the static fatiguecoefficient of the optical fiber is low. Therefore, it is desirable thatthe static fatigue coefficient of the optical fiber relating to thepresent invention be 20 or more. This will enable achieving desirablelong-term reliability of the optical fiber. Preferably, the staticfatigue coefficient of the optical fiber is 25 or more, and morepreferably 30 or more. Also, it is preferable that the breaking strengthof the optical fiber relating to the present invention be 400 kgf/mm² ormore when a tensile rate of 5 mm/min is applied.

The circumferential deviation of the compressive stress of an opticalfiber relating to the present invention is preferably 10 Mpa or less.This will allow a fiber curl to have a radius of curvature of 4 m ormore. The measurement of a residual stress in an optical fiber can bedone, for example, using the birefringence of the optical fiber asdescribed in Japanese Patent Application Publication No. 2009-168813.Or, the measurement of the residual stress in the optical fiber can alsobe made from the variation quantity of refractive index and photoelasticcoefficient inherent in the material by making a cross-sectionalanalysis of refractive indexes at a section of the optical fiber.

In the following, a manufacturing method for an optical fiber accordingto the present invention will be described. FIG. 6 is a conceptionalschematic diagram illustrating an embodiment of optical fibermanufacturing method of the invention. In an embodiment of optical fibermanufacturing method, first prepared is an optical fiber preform 4having a refractive index profile that is equivalent to thecross-sectional structure of an optical fiber to be produced. That is,the optical fiber preform 4 is a silica-based preform and includes acore, an optical cladding surrounding the core, and a jacketing regionsurrounding the optical cladding. The optical fiber preform 4 can beprepared by an arbitrary method such as the VAD method, the OVD method,or the MCVD method.

The optical fiber preform 4 is vertically put into a drawing furnace 41,and one end (bottom end) thereof is fused by heating in the drawingfurnace 41 so as to be drawn into a fiber. A glass filament 5 is formedby this fiber drawing step. The glass filament 5 has a desired outsidediameter that is the same as the glass diameter of an optical fiber tobe produced. At this fiber drawing step, it is preferable to performdrawing of the glass filament 5 by twisting it in alternate directionsabout the central axis thereof. By doing so, the circumferentialhomogeneity of the heating of the glass filament 5 will be increased ata later stress imparting step, and accordingly the given compressivestress will become circumferentially uniform.

The glass filament 5 formed at the fiber drawing step is inserted into astress imparting unit 42 after the temperature of the whole body hasbecome lower than the glass transition point. The glass filament 5inserted into the stress imparting unit 42 is re-heated there until thetemperature of the outermost circumferential portion of the jacketingregion becomes higher than the glass transition point, and as a resultof cooling this part, a compressive strained layer is formed at theoutermost circumferential portion of the jacketing region. By thisstress imparting step, an glass fiber 6 in which a residual compressivestrained layer is formed at the outermost circumferential portion of thejacketing region is produced.

The glass fiber 6 produced by the stress imparting step is coated withan ultraviolet curable resin at a die 43, and thereafter such coating ishardened at a curing unit 44, resulting in a primary coating. Inaddition, an ultraviolet curable resin is applied at a die 45 andthereafter is hardened at a curing unit 46, resulting in a secondarycoating. By this coating step, a coated optical fiber 7 is produced suchthat the circumference of the glass fiber 6 is covered with the primarycoating and the secondary coating. The coated fiber 7 produced by thecoating step is rolled up onto a bobbin 48 via a capstan 47.

Thus, an optical fiber of the present invention can be manufactured byan optical fiber manufacturing method of the invention. The opticalfiber manufacturing method of the present invention is such that thestress imparting step (the compressive strained layer is formed by thestress imparting unit 42) is only added to the latter part of the fiberdrawing step (the drawing is done by the drawing furnace 41) in aconventional optical fiber manufacturing method. With the method, anoptical fiber can be manufactured by drawing an optical fiber preform 4that is similar to a conventional preform. Therefore, the optical fibermanufacturing method relating to the present invention enables easilyaffording a compressive stress to the glass outer circumference withoutchanging the glass composition constituting the jacket layer of anoptical fiber, allowing easy and low-cost manufacture of the opticalfiber having excellent strength.

FIG. 7 shows a first example of stress imparting unit 42 used at thestress imparting step in the optical fiber manufacturing method relatingto the present invention: region (a) is a top view thereof; and region(b) is a side view thereof. In the first example, a compressive strainedlayer is formed in the following manner: a glass filament 5 is heated toa temperature higher than the glass transition point by means ofirradiation of laser beams output from lasers 51 to 53, and when thispart is cooled, the compressive strained layer is formed at theoutermost circumferential portion of the jacketing region. The number oflasers used in such case may be one, but it is preferable to use two ormore lasers. When a plurality of lasers are used, the heating of theglass filament 5 is equalized and accordingly the quantity of affordedcompressive stress is equalized around the outer circumference of theglass filament 5 since the laser beams output from the plurality oflasers are irradiated from different directions to the glass filament 5.

FIG. 8 shows a second example of stress imparting unit 42 used at thestress imparting step in the optical fiber manufacturing method relatingto the present invention: region (a) is a top view thereof; and region(b) is a side view thereof. In the second example, laser beams outputfrom the laser 51 are introduced into a tubular reflecting plate 54 viawindow 54 a and partly irradiated to the glass filament 5 directly whilethe rest of the beams are irradiated to the glass filament 5 throughreflection at the inner wall surface of the tubular reflecting plate 54.Preferably, the inner wall surface of the reflecting plate 54 has areflectance of 70% or more at the wavelength of the laser beams, and ismade of metal-plated quartz, alumina, metal, or the like. With suchcomposition also, laser beams are irradiated from various directions tothe glass filament 5, and consequently the heating is equalized aroundthe outer circumference of the glass filament 5 and hence the quantityof the compressive stress afforded thereby is also equalized. In thesecond example also, a plurality of lasers may be used.

FIG. 9 shows a top view of a third example of stress imparting unit 42used at the stress imparting step in the optical fiber manufacturingmethod relating to the present invention. In the third example, a laserbeam output from the laser 51 is introduced into the tubular reflectingplate 54 via the window 54 a and diffused by a diffuser 55.Subsequently, a part of the diffused beam is directly irradiated to theglass filament 5 while the rest is irradiated to the glass filament 5through reflection at the inner wall surface of the tubular reflectingplate 54. With such composition also, laser beams are irradiated fromvarious directions to the glass filament 5, and consequently the heatingis equalized around the outer circumference of the glass filament 5 andhence the quantity of the compressive stress afforded thereby is alsoequalized. Also, in the third example, a plurality of lasers may beused.

For the purpose of lasers 51 to 53 in the first to third examples,lasers capable of outputting a high-power infrared laser beam, such as aCO₂ laser or a copper-vapor laser, are suitably used, and also a laserwhich can output a CW laser beam is suitable. It is desirable to controllaser beam strength by monitoring the temperature of the outercircumference of the glass filament 5 so as to obtain a desiredtemperature, so that an optical fiber having stable quality can bemanufactured.

FIG. 10 shows a fourth example of stress imparting unit 42 used at thestress imparting step in the optical fiber manufacturing method relatingto the present invention: region (a) is a top view thereof; and region(b) is a perspective view thereof. In the fourth example, the glassfilament 5 is heated to a temperature higher than the glass transitionpoint by a heating furnace 61, and when this part is cooled, acompressive strained layer is formed at the outermost circumferentialportion of the jacketing region.

The heating furnace 61 is an annular furnace arranged to surround theglass filament 5 as its central axis, for example, such as Kanthalfurnace, a resistance furnace, an induction furnace, or the like. It ispreferable that the inside of the heating furnace 61 be filled with aninert gas such as N₂ gas, Ar gas, and He gas in a clean atmosphere. Theheating furnace 61 preferably has a length of 100 mm or more so that theoutermost circumferential portion of the glass filament 5 can be heatedto a temperature higher than the glass transition point. Also, it ispreferable to provide a mechanism for cooling the glass filament 5 thathas come out of the heating furnace 61 upon being heated by the heatingfurnace 61, so that a stronger compressive stress can remain at theouter circumferential portion of the glass fiber 6.

FIG. 11 shows a top view of a fifth example of stress imparting unit 42used at the stress imparting step in the optical fiber manufacturingmethod relating to the present invention. In the fifth example, theglass filament 5 is heated by a burner 71 to a temperature higher thanthe glass transition point, and when this part is cooled, a compressivestrained layer is formed at the outermost circumferential portion of thejacketing region. The burner 71 is one that can heat the glass filament5 to a temperature higher than the glass transition point (about 1100°C.), such as a plasma burner, an oxyhydrogen burner, or a methaneburner, for example. However, the flame is preferably an anhydrous one,because otherwise the long-term glass strength may not be assured if anOH group adheres to the surface of the glass filament 5. Also, from theviewpoint of preventing the glass filament 5 from shaking (line sway),the burner 71 is preferably a plasma burner that generates comparativelysmall wind pressure.

FIG. 12 is a graph illustrating a Raman scattering spectrum of silicaglass and 800 cm⁻¹ peak area A800 and 490 cm⁻¹ peak area A490 of thespectrum. The peak area A800 is an area of the region between thespectrum of Raman scattering and the baseline drawn from wave number 880to 740 cm⁻¹ and corresponds to the quantity of SiO₂ glass networkcomposed of six-membered ring. The peak area A490 is an area of theregion between the spectrum of Raman scattering and the baseline drawnfrom wave number 525 to 475 cm⁻¹ and corresponds to the quantity of astrained structure composed of four-membered ring.

A known model of glass breakage is such that strained structures such asthree-membered ring and four-membered ring that are generally inherentin the SiO₂ glass network composed of six-membered ring are selectivelyhydrolyzed, leading to breakage (J. K. West et al., “Silica fracturePart II A ring opening model via hydrolysis”, Journal of MaterialsScience 29 (1994) 5808-5816). Therefore, it is desired that the absolutequantity of three-membered ring and four-membered ring are lessened toincrease the breaking strength of glass.

FIG. 13 is a graph showing the ratio (A490/A800) at each position in asection of an glass fiber 6, in the case where the heat treatment of aglass filament 5 is performed and in the case where the heat treatmentof the glass filament 5 is not performed, respectively. (The spectrum ofRaman scattering at each position in the section of an optical fiber canbe measured by scanning pump light having a beam diameter of 5 μm in aradial direction in the section.) The ratio (A490/A800) at the outermostcircumferential portion of the jacketing region was made smaller thanthe ratio (A490/A800) at the core by performing a heat treatment at atemperature below (Tg+500° C.) and above the glass transition point Tg.In the case where a heat treatment was performed, the breaking strengthof the optical fiber in a tensile test at a strain rate of 1%/minincreased by 11% as compared with the case where no heat treatment wasperformed. As described above, it is possible to enhance the breakingstrength of the optical fiber by conducting a heating of the glassfilament 5 at a temperature below (Tg+500° C.) and above the glasstransition point Tg.

EXAMPLES

Three optical fibers (Example 1 to 3) were prepared experimentally asthe optical fiber 3 relating to Embodiment 3 by the optical fibermanufacturing method of the present invention, and opticalcharacteristics (mode field diameter at a wavelength of 1.31 μm cutoffwavelength, 22 m cable cutoff wavelength, zero dispersion wavelength,and bend loss at each bending diameter), residual compressive stressesin the optical fibers, and breaking strengths were obtained. As shown inFIG. 3, based on the refractive index of pure silica glass, the relativerefractive index difference at a first core 31 is expressed asΔ1=(n₃₁−n_(SiO) ₂ )/n_(SiO) ₂ , the relative refractive index differenceat a second core 32 is expressed as Δ2=(n₃₂−n_(SiO) ₂ )/n_(SiO) ₂ , therelative refractive index difference at a depressed region 33 isexpressed as Δ3=(n₃₃−n_(SiO) ₂ )/n_(SiO) ₂ , and the relative refractiveindex difference at a cladding 34 is expressed as Δ4=(n₃₄−n_(SiO) ₂)/n_(SiO) ₂ . The outside diameter of a first core 31 is expressed by2r₁, the outside diameter of a second core 32 is expressed by 2r₂, andthe outside diameter of a depressed region 33 is expressed by 2r₃. Ra isa ratio (r₁/r₂), and Rb is a ratio (r₂/r₃). The compressive stress wasmeasured with a technique for detecting polarization rotation byphotoelastic effect, and the average breaking strength was obtained byapplying a tensile rate of 5 mm/min to a testing optical fiber of 500 mmin length.

Table shows specifications of optical fibers in Examples 1 to 3.

TABLE Example 1 Example 2 Example 3 Refractive Δ1 % 0.3 0.35 0.35 indexΔ2 % 0.025 0.025 0.025 structure Δ3 % −0.7 −0.6 to −0.5 −0.7 Δ4 % 0.0250.025 0.025 r1 μm 4 4 4 r2 μm 6.6667 9.30233 8.16327 r3 μm 12.461115.5039 12.184 Fiber diameter μm 125 125 125 Ra (r1/r2) 0.6 0.43 0.49 Rb(r2/r3) 0.535 0.6 0.67 Properties MFD @1.31 μm μm 8.6 8.6 8.6 2 m cutoffnm 1400 1400 1400 wavelength 22 m cutoff nm <1260 <1260 <1260 wavelengthZero dispersion nm <1300 1300 to 1324 <1300 wavelength Bend loss @R5 mmdB/turn <0.1 <0.1 <0.1 Bend loss @R7.5 mm dB/turn <0.08 <0.08 <0.08 Bendloss @R10 mm dB/turn <0.03 <0.03 <0.03 Bend loss @R5 mm dB/turn <0.45<0.45 <0.45 Bend loss @R7.5 mm dB/turn <0.25 <0.25 <0.25 Bend loss @R10mm dB/turn <0.1 <0.1 <0.1 Compressive stress MPa 16 29 46 Breakingstrength kgf 5.62 5.95 6.35The optical fibers of Examples 1 to 3 had satisfactory opticalproperties, even if they were afforded with compressive stress at theirouter circumferential portion. In the case of an optical fiber in whichthe viscosity of the core is lower than that of the jacketing region, acompressive stress remains at the core after drawing. On the other hand,the compressive stress at the core decreases as a result of acompressive stress being afforded around the outer circumference. If anexcessive compressive stress is afforded to the outer circumference, atensile stress sometimes works at the core. When a compressive stressworks at the core, attenuation increases. Therefore, it is preferable toadjust heating conditions and tension, so that a compressive stress mayremain at the core while affording a tensile stress to the core.However, in the case of application at a short transmission distancewhere the attenuation does not matter, it is possible to strengthen anoptical fiber by increasing the compressive stress to the outercircumferential portion, without regard to the stress at the core.

FIG. 14 is a graph showing the relationship between a residual stressand a breaking strength in an optical fiber. In FIG. 14, additionalexamples are shown in addition to Examples 1 to 3. As shown in FIG. 14the larger the compressive stress at the outer circumference of thejacket, the better, because if the compressive stress is larger, thebreaking strength becomes higher.

The optical fiber relating to the present invention can have anarbitrary cross-sectional structure other than those of Embodiments 1 to4. Particularly, in an optical fiber having a trench-type profile or asimilar profile, less optical leakage occurs even if it is bent.Therefore, an optical fiber having such type of profile canadvantageously be used for increasing the efficiency of housing andinstallation work of FTTx because it is highly reliable in terms ofstress given during the installation work and storage.

Also, the optical fiber relating to the present invention can suitablybe used for submarine applications. Optical fibers installed on the seabottom are difficult to repair, and therefore they are required ofparticularly long-time reliability. Optical fibers relating to thepresent invention are preferable because they are highly reliable withrespect to stress. Optical fibers having a W-type refractive indexprofile of Embodiment 2 and those having a trench-type profile ofEmbodiment 3 are suitable for submarine applications.

The temperature of an optical fiber at the time of entering into the die43 for application of a resin is preferably 30° C. or higher. In suchcase, the condensation reaction between the glass and a silane couplingmaterial contained in the resin is accelerated, and hence the strengthof glass can be expected to further increase.

In the optical fiber manufacturing method relating to the presentinvention, the deviation of temperature at the outer circumference ofthe optical fiber during heating for the stress imparting step ispreferably less than 50° C. By heating in such manner, thecircumferential deviation of the compressive stress can be made equal toor less than 10 MPa. The temperature of an optical fiber under drawingcan be evaluated by observing the optical fiber under drawing from aplurality of angles with a pyrometer in which infrared light is used.

In the optical fiber manufacturing method relating to the presentinvention, preferably L1/V is equal to or more than 0.003 second, andL2/V is equal to or less than 1 second, where L1 represents the lengthto the stress imparting unit L1 from the position at which the outsidediameter of the optical fiber is decreased to 105% or less of desiredouter diameter, L2 represents the length of the stress imparting unit,and V represents the line speed of the optical fiber. It is possible tomake the temperature of the optical fiber to be below the glasstransition point by setting the cooling time of the optical fiber suchthat L1/V>0.003 second holds. In addition, by limiting the heating time,the viscosity decreasing part can be limited to the outer circumferenceof the optical fiber only, thereby enabling production of an opticalfiber in which the compressive stress is afforded only to the outercircumference of the optical fiber. More preferably, L2/V is 0.5 secondor less, and further more preferably, L2/V is 0.1 second or less.

In the optical fiber manufacturing method relating to the presentinvention, the tension given to the optical fiber glass at the stressimparting step is preferably 25 gms or more. The larger the tensiongiven to the optical fiber, the larger the compressive stress remainingat the outer circumference of the optical fiber. By so making thetension to be 25 gms or more, the compressive stress can be easilyafforded, enabling significant improvement in the strength of theoptical fiber over an optical fiber to which no compressive stress isafforded. More preferably, the tension is 50 gms or more.

As described above, the present invention makes it possible to provide aresidual compressive stress selectively only at the outside surface ofthe glass, notwithstanding an ordinary glass having a common glasscomposition is used as it is without adhering any particular material tothe outside surface of glass, provided that the glass composition hasbeen proved to be satisfactory in terms of optical properties. As aresult, because tensile stress given to the optical fiber is canceled bythe residual compressive stress, thereby restraining any flaw of theoptical fiber surface from growing, it is possible to obtain an opticalfiber in which its strength is enhanced. Thus, according to the presentinvention, it is possible to significantly improve the reliability ofthe optical fiber easily without having an adverse influence on themanufacturing cost while conquering the problems of the conventionalmethods that have been contrived to strengthen an optical fiber.

Thus, the optical fibers of the present invention can be used for wiringof FTTx or optical fibers installed on the sea bottom.

1. A silica-based optical fiber comprising a core, an optical claddingsurrounding the core, and a jacketing region surrounding the opticalcladding and having a uniform composition throughout from the internalcircumference to the outer circumference, wherein a compressive strainedlayer having a residual compressive stress is provided at the outermostcircumference.
 2. An optical fiber according to claim 1, wherein thestress of the compressive strained layer is a compressive stress of 10MPa or more.
 3. An optical fiber according to claim 1, wherein thecompressive strained layer has a thickness equivalent to 30% or less ofthe outer diameter of the jacketing region.
 4. An optical fiberaccording to claim 1, wherein the static fatigue coefficient is 20 ormore.
 5. An optical fiber according to claim 1, wherein the breakingstrength is 400 kgf/mm² or more when a tensile rate of 5 mm/min isapplied.
 6. An optical fiber according to claim 1, wherein the ratio(A490/A800) of 490 cm⁻¹ peak area A490 to 800 cm⁻¹ peak area A800 ofRaman scattering spectrum at the outermost circumferential portion ofthe jacketing region is smaller than the ratio (A490/A800) at the core.7. An optical fiber according to claim 1, wherein the circumferentialdeviation of the compressive stress is 10 MPa or less.
 8. A method ofmanufacturing an optical fiber, comprising: a fiber drawing step whereina glass filament having a desired outer diameter is formed by heatingand fusing one end of a silica-based optical fiber preform including acore, an optical cladding surrounding the core, and a jacketing regionsurrounding the optical cladding; and a stress imparting step whereinthe outer circumferential portion of the glass filament is re-heatedwith a stress imparting unit to a temperature higher than a glasstransition point after the temperature of the whole glass filamentformed in the fiber drawing step has become lower than the glasstransition point, and thereby a compressive strained layer is formed atthe outermost circumferential portion of the jacketing region.
 9. Anoptical fiber manufacturing method according to claim 8, wherein theglass filament is heated by means of irradiation of laser beams outputfrom one or more lasers.
 10. An optical fiber manufacturing methodaccording to claim 8, wherein the glass filament is heated by an annularheating furnace surrounding the glass filament as its central axis. 11.An optical fiber manufacturing method according to claim 8, wherein theglass filament is heated using a burner.
 12. An optical fibermanufacturing method according to claim 8, wherein at the drawing stepthe glass filament fiber is drawn while being twisted in alternatedirections about the central axis thereof.
 13. An optical fibermanufacturing method according to claim 8, wherein the deviation oftemperature in the outer circumference of the glass filament duringheating at the stress imparting step is less than 50° C.
 14. An opticalfiber manufacturing method according to claim 8, wherein L1/V is 0.003second or more, and L2/V is 1 second or less, where V is a line speed ofan optical fiber, L1 is a length extending to the stress imparting unitfrom the position at which the outer diameter of the glass filament isdecreased to 105% or less of the desired outer diameter, and L2 is thelength of the stress imparting unit.
 15. An optical fiber manufacturingmethod according to claim 8, wherein tension applied to the opticalfiber glass at the stress imparting step is 25 gms or more.